If your goal is building muscle and improving maximum strength
Several studies have shown that muscle and strength-building suffer when you combine endurance training with strength training. A good warm-up routine will activate your body, and bring it up to operating temperature. 5-10 minutes on an ergometer are all it takes. However, if you make your workout an intensive endurance session, not only is your body warmed up, but it also puts a strain on your body as well. This can lead to a situation, in which your muscles are no longer subjected to a sufficiently intense load, and fail to trigger an optimal and effective training stimulus. In addition, pre-fatigue worsens coordination, and weakens your trunk stabilising muscles, increasing the risk of injury. Therefore, effective strength training should never be carried out with pre-fatigued muscles.
A long, endurance session even directly after strength training is not recommended, as this can negatively influence the muscle-building processes. In the following meta-study, in which 21 studies on the topic “muscle build-up and endurance training" were compared and put into relation, it was agreed that treadmill training has a considerable influence on the body’s potential for muscle growth. Already 30 minutes and more on the treadmill will lead to significant losses in both the body's strength level and hypertrophy potential. The longer the endurance training lasted, the more significant the results were. For this reason, cardio and strength training should be done at separate times or, even better, on separate days. This is likely to result in less lost muscle mass, and can also improve your cardiovascular system.
If you want to improve endurance while running
If your main focus is on improving endurance, then you should complete your running session before beginning your strength training. It is important, especially during long and/or intense running sessions, that your muscles are fully fit and regenerated. However, if these are pre-fatigued, then this will have a negative effect on their economy of movement, and your running technique. This can lead to misuse and overstraining of your joints and muscles. If you still want to run after completing your strength training, then only choose a low intensity workout instead. In other words, a short endurance run in a low intensity, basic endurance range.
If your goal is to lose body weight
It is often recommended to do an endurance session after strength training to deplete your glycogen stores. The body is forced to extract more of its energy from fats in the subsequent running session. The problem with that is: Full efficiency cannot be achieved with depleted glycogen stores, and even prolonged intense activities cannot be sustained. Even though the percentage of fat burning during energy production is very high, calorie consumption is relatively low due to the low training intensity.
As mentioned before, a negative energy balance is the most important factor in weight loss: If you have burned more calories than you actually need, you lose weight. What counts at the end of the day is the total number of calories burned by the training. It is therefore advisable to divide your sessions across separate days. This ensures that you are able to exercise intensely, that you burn many calories, and that your body has enough time to regenerate adequately.
Your personal objective determines which order makes the most sense. The first training session should always correspond to your main focus. Whoever wants to focus on both aspects should plan their strength and endurance training for different days or complete their cardio in the morning and their strength training 7 in the evening, for example. This way, you can guarantee that you can achieve peak performance and don’t overload yourself in both sessions!